Combination radio-recording-communication system



July 28, 1942. J. H. REBOLD 2,291,105

COMBINATION RADIORECORDING-COMMUNICATION SYSTEM Filed March 20, 1941 2 i5 R'F'.

' AMR INVENTOR Patented July 28, 1942 COMBINATION RADIO-RECORDING-COMMUNICATION SYSTEM Jacob H. 'Itcbold, Brooklyn, N. Y. ApplicationMarch 20, 1941, Serial No. 384,371

20 Claims, (01. ire-1310.11)

- the machine as they do during the comparatively This invention relatesto a combination radiorecording-communication system, and moreparticularly to-a system normally adapted to receive and record radiobroadcasts, but having locally or remotely controlled means fordisabling the system for radio reception and selectively conditioning itfor line communication with the remote point or for reproducing and Irecording from microphone r pickup.

Well-known to the art are combination radiophonograph-recording machineswhich, under the control of a circuit switch mounted on the machine andmovable to a number of positions corresponding to the number ofdifferent functions, can be selectively conditioned by the operator toperform any of the following functions:

1. To receive radio programs which are audibly reproduced by means of aloud-speaker.

2. To record radio programs on a suitable record blank, with or withoutan accompanying sound reproduction by means of a loud-speaker.

' 3. To reproduce or play-back records electrically by means of aloud-speaker, the machine being disabled for radio reception meanwhile.

4. To record local selections picked up by a local microphone, with orwithout an accompanying sound reproduction by means of a loudspeaker.

5. To audibly reproduce in amplified relation by means of aloud-speaker, local programs or announcements picked up by a localmicrophone.

Such machines usually include a radio-frequency amplifier and detectorfor the radio re ception and recording functions, followed by avolume-control, audioamplifier, and loud-speaker common to all'of thefunctions. During normal reception of powerful broadcast carrier waves,the radio-frequency amplifier and detector usually deliver to thevolume-control signal potentials averaging several volts, whereas thelocal a point where it will satisfactorily operate the recording deviceor the loud-speaker.

To supply this extra microphone amplification, one or more additionalelectron tubes are usually added to the complement requiredfor radiocurrent and age during any other function of reception only, and theseadditional tubes draw limited time that they are actually at workamplifying the microphone potentials, since no provision is generallymade for disconnecting the heaters ofthese extra tubes during thosefunctions when the microphone is not in use.

In addition, when only a single pre-amplifier tube is provided, theoutput level of this tube,

while perhaps suflicient to operate the common audio-frequency amplifierto drive the recording tool or loud-speaker when the commonvolumecontrol is set at or near full volume, is still considerably belowthe level of the average rectified radio carrier, so that continual andcareful manipulation of the volume-control as well as of the change-overswitch is necessary in changing from one function to another; If therecord reproducing pick-up is also arranged to be connected across thecommon volume control, it too will likely have an output levelsubstantially different from that of either the radio detector or themicrophone, further complicating the problem of controlling the variousinput voltages to a suitable operating level. Should the operatorneglect to turn back the volume-control before switching from microphonerecording to radio reception, for example, a terrific blast would beheard from the loud-speaker, or if the recording unit remains connectedto the amplifier output, it may be permanently damaged by the excessi-vevoltages across it.

One object of the present invention is to provide a system of thegeneral type hereinbefore described, including a dual-function electrontube and circuit therefor capable of acting as .both a radio-frequencyor intermediate-frequency amplifier and an audio-frequency preamplifierfor boosting the level of a microphone or other pick-up, thus obviatingthe necessity v for including extra tubes and circuits for the latterpurpose.

Another object of this invention is to provide circuit means forcoupling a dualfifuncticn highand low-frequency amplifier t6 a detectorand common audio amplifier, whereby either highfrequency signals may beamplified and demodulated, or low-frequency signals may be amplifiedonly, without requiring switching of said circuit means.

A further object of my invention is to provide means for combining ormixing the various in- .puts from radio, microphone, and pick-up to thecommon volume-control and audio amplifier so that a given adjustment ofthe volume-control will result in a substantially equal loudspeakervolume from any of'the difierent input Referring to Fig. 1,. asuitablereceiving antenna l is connected to the input terminals of aradio-frequency amplifier 2, which in turn is connected tofirst-detector and mixer stage 3, to which is also connected thehigh-frequency oscillator 4, part of which is the oscillator tube 5. Themixing of the locally generated high-frequency current with the incomingreceived radioprovide novel means for simultaneously disabling' the.radio receiving portion of the system and operatively connecting amicrophone or pickup thereto. I

Another obiect'of my invention isto provide novel means, locally orremotely controlled, for

simultaneously disabling the radio receiving portion of the system andoperatively connecting thereto a locally-or remotely located microphoneor pickup.

Another ,object of this invention is to provide a plurality ofradio-recording-communication stations, normally adapted to receiveradio broadcast programs, with means by which any station may disablethe radio receiving portion of another station andcommunicate therewithor send program material for recording thereto, and which may in turn besimilarly conditioned by remote control from the other station, thusailowing a two-way intercommunication between the two stations.

It is also an object of my invention to provide one or moreradio-recording-communication stations, normally adapted to receive andreproduce radio programs, with improved means whereby a microphone orother pickup may be connected with any one station, and through the'operation of a single-circuited switch means carried by the microphone,will simultaneously disable that station for radio reception and placeit in condition to amplify and reproduce or record sound currents pickedup by the microphone.

Another object of the present invention is to provide' aradio-recording-playback system with improved and simplifiedthree-position switching means permitting the function-selecting controltobe manually operated or automatically controlled by movement of therecording. and playback mechanism into position, in conjunction withswitch means carried'by a microphone allowing instantaneous cut-in andcontrol of sound reproduction or recording by the operator of themicrophone.

Supplementary objects and advantages of my invention will be apparentfrom the remainder of this specification and from the annexed drawing,in which:

Fig. 1 is a generalised circuit diagram indicat- I ing one form of myinvention using separate recording and reproducing units, theconventional elements of the circuit familiar to those skilled in theart being indicated in block form to avoid.

Fig. 4 is a partial schematic circuit diagram indicating severalmodifications which may be made in the radio-disabling circuit and inthat of the function-changing circuit switch.

frequency current in stage 3 is accomplished in a manner well-known tothe art, and results in an intermediate-frequency current which isapplied to the intermediate-frequency amplifier 6, and in turn tothefinal intermediate-frequency stage I. The intermediate-frequency outputof stage 1 feeds into second-detector and rectifier stage 8, which in awell-known manner converts the amplified intermediate-frequency voltagefed to' it into a direct-current automatic-volume-con- -trol voltagewhich may be fed back along AVC connection 9 to automatically controlthe gain of any or all of stages 2, 6, and 1. In addition,second-detector stage 8 delivers to the outer terminals ofvolume-control III an audio-frequency voltage representative of themodulation originating upon the incoming radio-frequency current orcarrier wave picked up by antenna I. The movable contact I] onvolume-control l0 serves to control the amount of the audio-frequencyvoltage fed to the input of audio-frequency amplifier l2, the amplifiedoutput of which in turn feeds into the speaker coil l3 which operatesloud-speaker ll to audibly reproduce the translated audio-frequenciesrepresentative of the original carrier modulation.

To the output terminals of audio-frequency amplifier l2 may be connectedthe recording unit l5 through the controlling switch I 6, which uponbeing open-circuited will operatively disconnect unit l5, and may becombined with other switch means to disconnect or reduce the volume ofloud-speaker ll when unit I 5 is in operation. In the diagram, recordingunit I5 is indicated as being of the electro-magnetic type, but may alsobe of any of the other types suitable for the purpose and well-known tothe art, such as piezoelectric, etc.

Microphone I! and pickup I8 are indicated in the diagram as being of thepiezo-electric or crystal type, but may likewise be of other well-knowntypes of inherent high impedance, or made so through a suitable couplingtransformer. Microphone I1 may conveniently carry within its case I! theblocking condenser 20 and the microphone switch 2|, which latter may bea singlecircuit toggle or push-to-talk switch which opencircuits when itis desired to use the microphone. Switch 2| may be connected asindicated across the microphone leads 22, which is the high-potentiallead, and 22, which is the grounded lead and may conveniently take theform of an encircling shield or braid around the suitably insulated lead22 to protect it from external electrical disturbances.

Pickup I I8 is also supplied with a pickup switch 24, which may beconnected across its leads as s hown and will serve to place the pickupinto condition to be used when the switch is open-circuited. 25 and 26represent the input terminals for the pickup l8, and may beshort-circuited as indicated in the diagram by the dotted lines, whenpickup II is removed from the circuit.

Oscillator tube 5 may have its control grid 21 coupled to oscillator 4by means of the grid condenser 22, arid grid resistor 29 serves tocontrol the direct-current bias of the control-grid 21,

being normally adapted to connect with oscillator cathode 30 when thecircuit is conditioned for radio reception. Interposed between gridresistor 23 and oscillator cathode 30, however, is connected the sourceof direct potential 3|, its positive pole connected with cathode 30, itsnegative pole connected to the return end of grid resistor 29 throughresistor 32. Microphone switch 2| and pickup switch 24 may be seriallyconnected as shown across the junction point of resistors 29 and 32 andcathode 30, so that when both switches are closed, grid resistor 23 willbe returned directly to cathode, allowing oscillator 4 to functionnormally. The value of potential 3| is made high enough so that, wheneither switch 2| or 24 is opened, the additional negative potentialplaced upon control-grid 21 will. block oscillator tube 5, causing itand oscillator 4 to positively cease oscillation. When this happens, nointermediate-frequency voltage is produced by mixer stage 3, and thesystem is inoperative insofar as reproduction of received radio programsis concerned. The impedance of resistor 32 is made high enough so as notto appreciably load microphone H or pickup l8 when'their respective.control switches are open, and so as to protect potential source 3| whenboth switches are closed.

Final intermediate-frequency amplifier stage I may include the inputintermediate-frequency transformer 33, feeding into control-grid 34, andoutput intermediate-frequency transformer 35, feeding out of plate 36 ofamplifier tube 31, all in a conventional manner. The grid return end oftransformer 33 may be connected through resistor 38 to the AVGconnection 3, and also through condenser 39 to the microphone-pickupinput terminal 25, which is normally grounded when switches 2| and Hereclosed. Thus condenser 39 acts as control-grid-return bypass to thecathode of amplifier tube 31 when input terminal 25 is grounded, and asaudio coupling con denser to control-grid 34 for microphone I! or pickupI8 when switch 2| or 24 is open, the secondary of input transformer 33presenting a negligible impedance to audio-frequency currents flowingthrough it.

Amplifier tube 37 has at least one additional grid electrode 40 locatedbetween control-grid 34 and plate 33, which is biased positively throughresistor 4| connected to the high-potential plate power supply. Whenamplifier tube 31 is functioning as an intermediate-frequency amplifierin the usual manner, grid 40 acts as a screengrid electrode, bypasscondenser 42 serving to hold it effectively at ground potential forthese frequencies. The value of condenser 42 is made large enough toeffectively bypass high-frequency currents, and yet small enough so asnot to appreciably attenuate audio-frequency currents which may appearupon grid 43, a suitable value for which may be in the neighborhood of200 micro-microfarads.

Grid 43 may be connected, with tap 43 on vol- -ume-control Hi throughresistor 44 and couplingcondenser 45. Resistor 44 in conjunction withbypass condenser 42 forms a low-pass filter effectively preventingintermediate-frequencyvoltage originating on grid 40 from reachingvolumecontrol Iii, yet permitting audio-frequency voltages to passthereto.

Tap 43 may be so located intermediate the total resistance ofvolume-control l0 that the portion 46 between tap 43 and low potentialand 41 has a resistance value whose ratio to the total resistance valueof volume-control I0 is substantially equal to the ratio of averageaudio-frequency voltage delivered by grid 40' when amplifying microphonevoltages, to the average audio-frequency voltage delivered bysecond-detector stage 3 when radio programs are being received. Theactual physical position of tap 43 is placed close to the upper end ofvolume-control III; for example with an angular range for the totalresistance of 270 degrees, the lower portion 48 may occupy an angularrange of about 225 degrees, or approximately percent of the totalrotation of travel of movable contact H. In this manner, a substantiallyequal audio input is 'fed to audio-frequency amplifier |2 for agivensetting of movable contact upon lower portion 48 ofvolume-controllll, in spite of switching the audio source to microphone,pickup, or rectified radio carrier of moderate strength. For weak radiocarrier waves, movable contact Ii may be moved into contact with anyportion of the remaining upper part of volume-control I0 above tap 43,but for most requirements will remain below this point.

From the foregoing description and from Fig. 1, it will be apparent thatamplifier tube 31 can act to amplify either audio-frequency orhighfrequency voltages applied between control-grid 34 and ground, theamplified high-frequency voltages being taken out between .plate 36 andground in the usual manner, and the amplified audio-frequency voltagesbeing outputted between screen-grid 40 and ground. The audiomagnitude,in conjunction with the usual gain of audio-frequency amplifier I2, isgenerally more than sufiicient to raise the output level of microphone Hto a, point where it will satisfactorily operate loud-speaker l4 orrecording unit i5, thus making unnecessary the addition of an externalmicrophone pre-amplifier.

With both switches 2| and 24 closed and switch l6 open, the system is incondition to reproduce radio program material audibly throughloudspeaker M in the usual manner. Closing switch I6 willpermit'recording unit IE to recordthis radio program material upon asuitable record blank. Opening switch 2| will simultaneously andautomatically disable oscillator 4 and operatively connect microphone l1to the input of amplifier tube 31, and if switch l6 remains closed,

program material picked up by microphone Il may. be recorded byrecording unit l5. Since 7 control switch 2| is mounted on themicrophone, and may be located at a distance from the main portion ofthe amplifying apparatus, the operator of the microphone may at anymoment break into a radio program being recorded to make an announcementor transmit other material, and instantly bring back the radio programby closing switch 2| when the announcement is concluded. If switch I6 isopen, opening switch 2| will allow the microphone operator to break inupon the audible loud-speaker reproduction of the radio program to makeamplified announcements through loud-speaker l4. Again closing switch 2|and opening switch 24 will connect pick-up unit I! to the input ofamplifier tube 31, permitting amplified reproduction of records throughloud-speaker l4, the radio receiving portion of the system beingdisabled.

Referring now to Fig. 2, which shows a more detailed commercialembodiment of one form of my invention, antenna now feeds into antennacoil 48, which in turn is connected to the controlgrid of converter tube49, which in this case replaces stages 2, 3, and 5, and oscillator tubeof Fig. 1, acting as combined radio-frequency amplifier, oscillatortube, mixer, and intermediate-frequency amplifier, in a mannerwellknown' to the art. The output plate of'converter tube 49 feeds intoinput intermediate-frequency transformer 33, which is connected tocontrolgrid 34 of amplifier tube 31, which is similar to tube3l of Fig.l, but has an additional suppressor-grid located near plate 35 andconnected to cathode 50, which may be grounded as shown. The grid-retumterminal of transformer 33 is connected to the automatic-volume-controllead 5 through resistor 38, which may have a value of 1 megohm, and tothe movable switch-arm 5| through condenser 39, which may have a valueof 0.01 microfarad. Screen-grid electrode "of amplifier tube 31' isbypassed for intermediate-frequencies by condenser 42, and connected foraudio-frequency transmission to volume-control tap 43 through resistor44 and coupling condenser 45, which may have values of 0.1 megohm and0.01 microfarad, respectively. Screen-grid 40 in addition is positivelybiased through resistors 4| and 52 serially connected to the highvoltageplate supply lead B+, While condenser 53 connected between the junctionpoint of these two resistors and ground forms with resistor 52 anaudiofrequency decoupling filter. Suitable values for resistors 4| and52 and condenser 53,

-outputs substantially equalized for different portions should remainabout 1 to 3 for optimum might be 0.25 megohm, 0.05 megohm, and 4plifier tube 55, which in a well-known manner rectifies theaudio-modulated intermediate-frequency voltage and converts it into anaudio-irequency componentand adirect-current component, both of whichappear across the diode load resistor 55. The direct-current componentmay be utilised to automatically control the gain of mixer tube 45 andthat of amplifier tube 41' through AVC lead 9 connected with the gridre-' turn leads of these tubes, with potential source 58 supplying aresidual negative grid bias' when no radio carrier is being received.The audiofrequency component is fed to volume-control through couplingcondenser 51, while movable contact II feeds a predetermined portion ofthis component to audio-frequency grid 59 through coupling condenser 55.The amplified audio-irequency voltage appearing on plate 5| i fed to theinput electrodes of power amplifying tube 52 through the elementsindicated in the diagram, in a manner well-known to the art. The plateoutput circuit of power tube 52 may be coupled with loudspeaker |4through speaker coupling transformer 53, and is also connected toswitchterminal 54 as indicated.

Volume-control I0 is divided-by tap 43 into two portions, 45 and 55. Byso dividing the vol-' results. Although the ohmic resistance of poretion 55 is considerably greater than that of portion 45, it shouldoccupy a considerably smaller physical portion '.of the total activeresistance length, or travel of movable contact As previously suggested,portion 55 should extend for about 15 percent of the total controllength, with portion 45 taking up the remainder. In this mannerpractically the full control area can be used for control of both radioand local programs, it being" seldom necessary to bring movable contactonto portion 55, the over-all gain of the system for radio carriersbeing great enough to permit full power output with a greatly reducedsetting of the volume-control, except on the weakest of carrier waves.When it is necessary to use the full amplification of the system,however, this can be realized by moving contact onto portion 55, ortothe full-on position where it connects directly with diode couplingcondenser 51. Rotary volume-controls of the composition resistance type,whose resistance per unit of rotation may be tapered or varied toconformwith the requirements outlined above, are wellknown to the art,and further description of the mechanical features of volume-control I5is deemed unnecessary.

Oscillator tank tuning circuit, may be coupled to the oscillator gridand oscillator anodegrid electrodes contained within mixer tube 45 bycondensers 25 and 51, respectively. Oscillations of a frequencydependent upon the constants of the tuning circuit may be generated in awell-known manner through the capacitative coupling afforded by paddingcondenser 55. The oscillator grid resistor 29, which in the usualoscillator circuit would be returned directly to the cathode of mixertube 49, in this case is connected to the movable switch-arm 5|, towhich point is also connected the resistor 59, which latter is alsoconnected through resistor 15 to the negative potential terminal 1|. Tosuppress potential fluctuations which might originate in the externalpotential source, filter condenser 12 may be connected between groundand the junction point of resistors 55 and I5. Suitable values forresistors 55 and 15 and condenser 12 may be 5 megohms, 5 megohms, and0.25 microfarad, re-

spectively.

Terminals l3 and II are to be connected with a suitable source ofhigh-potential capable of ume-control, not only are. the audio-frequencysupplying the space current for the various electron tubes, and may be abattery, direct-current power line, or rectified alternating-currentsupplied power unit of a well-known type. Between the negative terminalII and ground may be connected the impedance I4, which may be a chokecoil or loud-speaker field which, in conjunction with filter condensersl5 and 15, forms a filter The individual values of theseelectricaloscillations representative of sound into' corresponding mechanicalstylus vibrations or vice-versa, may be connected through blockingcondenser 18 to movabl switch-arm l9 and switch-contact 80. Fromswitch-contact 8| to the grounded return lead of unit 11 may beconnected the attenuating condenser 82, the function of which is toreduce the normal output of unit ll, when it is connected for playbackor reated byoperatively applying unit TI to a record will be audiblyreproduced by loud-speaker l4 production of records, to a levelsubstantially equal to that of microphone II. It is well known that acapacity in parallel with a-piezo-electric type of transducer willreduce its output level in direct proportion to the ratio of the valueof the capacity to the effective capacity of the piezoelectric unititself, without frequency discrimination. If an electromagnetic type oftransducer is used, attenuating condenser 82 may be replaced by a fixedresistor attenuator, which will accomplish the above described functionof reducing the playback output, in a manner wellknown to the art.Suggested values for condensers I8 and 82 are 0.01 and 0.1 microfarad,respectively, but these values may be varied considerably.

Microphone IT, as previously described, is preferably mounted in acasing l9 which may carry the single-circuit control switch 2| and theaudio-coupling condenser 28. The purpose of the latter is to block outthe direct-current potential which is present across switch 2| when itis open, but it is not absolutely essential, and may even bebeneficially eliminated when a type of microphone is used which requiresa polarizing potential across it, such as the condenser or electrostatictype. Lead 22 from microphone I1 is connected with both switch-contacts83 and 84, while grounded lead 23 may conveniently form a shield aboutlead 22.

Function-control switch 85 may be of the rotary 3-position type withmovable arms 5| and I9 movable simultaneously through a single controlmember. It may be mounted on the chassis carrying the amplifying andradio receiving apparatus and controlled by hand, or it may be mountedin such position as to be automatically operated by movement of unit 11into recording or reproducing position. Such an automatically controlledswitch is shown in my co-pending application,

Serial Number 380,462, and switch 85 and its associated connections mayreadily be adapted to the construction therein described.

With switch 85 in the central or playback position as indicated in Fig.2, oscillator grid resistor 29 and audio-coupling condenser 39 are bothremoved from their normal ground connection, the oscillator grid ofmixer tube 49 is blocked by the additional negative bias,- and ashereinbefore explained, audible reproduction of radio programs isprevented. Unit 11. is now coupled through condensers" and 39 and thesecondary of transformer 33 to the control-grid of amplifier tube 31',and audio voltages generin a manner previously described. Microphone I1is disconnected in the playback position, and manipulation of switch 2|will have no effect on the circuitoperation. Thus records may be playedwithout interference from radio or microphone break-in.

Shifting movable arms 5| and 19 to a position to the left of thatindicated in Fig. 2 will condition the system for radio reception,assuming microphone switch 2| is closed. Both oscillator grid resistor29 and condenser 39 are grounded through switch 2|, the extra negativebias is grounded out, allowing normal oscillator action and radioreception, and condenser 39 resumes its normal function as AVC bypassfor amplifier tube 31'. Unit 11 is grounded out of thecircuit,preventing possible damage.

Opening microphone control switch 2| while switch 85 is in the left-handor radio position will open the ground connection of oscillator'gridresistor 29 and condenser 39, interrupting the switch 85 is in thelast-described recording posiaudible reproduction of radio programs byloudspeaker I4, and simultaneously operatively connecting microphone Hto the amplifying system to permit the loud-speaker to audibly reproduceannouncements orprogram material picked up by the microphone. Reclosingswitch 2| will instantly restore the original radio program.

Shifting movable arms 5| and 19 to a position to the right of thatindicated in Fig. 2 will connect unit 'l'l to the output circuit ofpower tube 62 through the blocking condenser 18. microphone switch 2|again closed, unit TI will be conditioned to record the radio programbeing received on a suitable record blank in a manner well-known to theart, the program being also audible through loud-speaker M.

Opening microphone control switch 2| while tion will again interrupt theradio program and permit microphone announcements and program materialto be recorded, either exclusively, or as a portion of a radio programrecord. If-microphone I1 is located near loud-speaker l4 and J there isdanger of acoustic feed-back between them, additional switch contactsmay be added to switch to disconnect loud-speaker M or operate it atreduced volume, in a manner wellknown to the art. during the recordingoperation. From the preceding description it is apparent that I haveprovided a simplified three-position function-switch circuit, which inconjunction with an auxiliary on-ofi switch preferably carried by themicrophone, offers a system of great flexibility, capable of performingat least five distinct functions. The function-switch may be eithermanually or automatically operated, the microphone switch may be eitherlocally or remotely located, the disabling of the radio receiver whenrequired is instantaneous and complete, and no switching of the audiocircuitsis required during any change of function. The greatlysimplified switching requirements make possible the use of a simple andinexpensive single-deck control switch with a minimum of leadconnections between it and the rest of the system, making it possible tomount it at any convenient point. For a given setting of thevolume-control, a substantially equal power output is obtained,

' regardless of change of input source, and degen- With - push-to-talkswitch, without the interposition of mechanical relays or othercomplicated equipment. In addition, for microphones or other types ofgenerators requiring a polarizing potential, means are provided forsupplying such a.

potential through the same elements which serve to disable the radioreceiver, making unnecessary the furnishing of additional means for thispurpose.

Although I have illustrated in Figs. 1 and 2 the application ofmyinvention to a radio receiver of the superheterodyne type, I wish itto be understood that it is equally applicable to other well-known typesof receiving circuits. In the so-called tuned-radio-frequency type ofreceiver, for example, the tube which would replace the dual-frequencyamplifier tube 81 would.be the final radio-frequency amplifier tube, 1.e., the tube feeding into the detector. The radio-dis- T abling negativepotential could be applied through a suitable grid resistor to thecontrolgrid of the next-preceding radio-frequency amplifier tube, theadditionalnegative bias applied, were the microphone switch opened,being sufficient to block this grid and prevent transmission of radiosignals beyond this point. Many other circuit variations will beapparent to persons skilled in the art, and therefore I do not wish myinvention to be restricted in its scope to the specific forms shown,except insofar as required by the prior art and the range of theappended claims.

-'Referr ing to Fig. 3, I have there indicated how a plurality ofsystems such as those shown in Figs. 1 or 2 may be interconnectedthrough a plurality of switches, I88 and 288, to provide aselectivetwo-way intercommunication system. In the drawing the prefix Iis added to the referencenumerals of the various elements at station Iwhich are similar to elements shown in Figs. 1 and 2, and likewise,similar elements at station 2 have their reference numerals prefixed bynumber 2. Only a portion of the microphone input circuit of each stationis indicated, it being understood that casings Iilll and 288 represent acomplete system such as shown in Figs. 1 or 2, except for themicrophones, which are indicated separately. Switches I88 and 288 may besimilar double-pole double-throw types, preferably of the rotaryvariety. On'e switch is located at each station, and they may beconnected as indicated, between the shielded cables interconnecting thestations.

with both of switches I88 and 288 in the position shown in Fig. 3, andwith both microphone switches III and HI closed, both stations areconditioned to receiveradio programs, but each is susceptible to beingcalled by the other. That is, if microphone switch I2 I at station I isopened, station I will continue to receive radio programs, but those ofstation 2 will be interrupted,

up by microphone ill will be audibly reproduced by the loud-speaker atstation 2. This material may also be recorded at station 2 by throwingits function control switch to the recording position. Likewise, openingmicrophone switch I will permit microphone 2" to control the loudspeakerand recorder of station I, and if both microphone switches are open atthe same time, a running two-way. conversation or exchange of programmaterial may take place between the two stations, without requiringfurther manipulation of switches to "talk or "listen."

If either station desires privacy, or does not wish to be interrupted bythe other station, the

double-pole switch at that station (I88 or 288), is thrown to theopposite position to that shown in Fig. 3, whereupon each microphone andswitch is connected with its own station, and is ineffective to controlthe other. Throwing either one or both of switches I88 and 288 willaccomplish this.

By eliminating switches I88 and 288 in Fig. 3 and connecting eachmicrophone and its control switch to the other station-directly, eachwill be under the control of the other at all times that the systems arein operation. It is likewise possible under these conditions toeliminate the recording function at each station,

leaving a plurality of normally radio-receiving but care must beexercised to avoid microphonic action between the open cathode, which isconnected with the input of the amplifier, and ground. In this figure isalso indicated how the microphone switch may be taken away from themicrophone casing and combined with the function-selecting switch. whichwould then have as many positions as the number of functions required ofthe system, in this case five. The remote control feature would besacrificed, but for some applications it might be advantageous to use asingle chassis-mounted function-control switch. 1

I claim asmy invention:

1. In amplifying apparatus, an amplifier including an electron tubehaving at least a cathode, a plate electrode, a control-grid electrode,and an auxiliary electrode, a signal input circuit coupledwith a sourceof high-frequency currents, a source of low-frequency currents, a

high-frequency output circuit for said electron tube connected betweenthe plate electrode and the cathode thereof, a low-frequency outputcircuit permanently connected between the auxiliary electrode and thecathode of said electron tube, and circuit means connecting said sourceof low-frequency currents and said signal input circuit in seriesrelation between the control-grid electrode and the cathode of saidelectron tube.

2. The combination as defined in claim 1, together with means forselectively short-circuiting the said source of low-frequency cur rents.

3. The combination as defined in claim 1, together with means forselectively rendering ineffective the said source of high-frequencyourand announcements or program material picked rents.

4. The combination as defined in claim 1, together with selective meansfor alternately disabling either the said source of low-frequencycurrents, or the said source of high-frequency currents.

5. In a dual-frequency amplifying system, an electron tube having atleast four electrodes including a cathode, a plate electrode, acontrolgrid electrodes, and an auxiliary electrode, a source ofmodulated carrier voltages and a separate source of audio voltagesconnected with the input circuit of said electron'tube between thecontrol-grid electrode and the cathode thereof, a volume-controlresistor having terminal end connections, a tap connection intermediateits ends, and a variable connection movable between its ends and pastthe tap connection, a moduletion detector having a carrier-frequencyinput circuit connected between the plate electrode and the cathode ofsaid electron tube and an audio output circuit connected between theterminal end connections of said volume-control resistor, an audioutilization circuit connected between the variable connection and oneterminal end connection of said volume-control resistor, and an audiotransmission circuit connecting the auxiliary electrode and the cathodeof said electron tube with the tap connection and said one terminal endconnection, respectively, of said volume-control resistor.

, 6. The combination as defined in claim 5, together with means forselectively short-circuiting the said separate source of audio voltages.

7. The. combination as defined in claim 5, characterized in that thesaid source of modulated carrier voltages and the said separate sourceof audio voltages are serially connected with the input circuit of thesaid electron tube between the control-grid electrode and the cathodethereof, together with means for selectively s'hort-circuiting the saidsource of audio voltages.

8. The combination as defined in claim 5, characterized in that the saidaudio transmission circuit between the auxiliary electrode of the saidelectron tube and the tap connection of the said volume-control resistorincludes a resistor and a capacity serially connected therein. 9. In acombination radio-phonograph system, a modulated carrier wave receiverincluding a modulation detector having an audio output circuit, anamplifier including an electron tube capable of amplifying audiovoltages and having audio input and output circuits, a source of audiovoltages connected with the input circuit of said amplifier, avolume-control resistor effectively connected in shunt with the audiooutput circuit of said detector, circuit means effectively connectingthe audio output circuit of said amplifier in shunt with a predeterminedfixed portion of said volume-control resistor, and an audio utilizationmeans eifectively connected in shunt with a manually variable portion ofsaid volume-control resistor.

10. The combination as defined in claim 9, characterized in that theohmic value of the fixed portion of the said volume-control resistorwhich is effectively connected in shunt with the audio output circuit ofthe said amplifier, is less than one-half of the ohmic value of the saidvolumecontrol resistor which is effectively connected in shunt. with theaudio output circuit of the said detector.

11. In a combination radio-phonograph system, a modulated carrier wavereceiver including an electron tube having at least an anode, a cathode,and a grid electrode adapted to control the space current thereof, asource of blocking potential connected to said grid electrode andcapable of biasing it beyond cutoff, thereby to render said electrontube ineffective, an amplifier capable of amplifying audio voltages andhaving input and output circuits, audio utilization means connected tothe audio output circuit of said amplifier, a source of audio voltages,circuit means connecting said source of audio voltages with saidamplifier input circuit and effectively in shunt with said source ofblocking potential,

and selective switch means capable of efiectively short-circuiting bothof said sources simultaneously, thereby rendering said sourcesineffective to block said electron tube or to control said amplifier.

12. The combination as defined in claim 11, characterized in that thesaid selective switch ,means comprises a single-pole switch devicecapable of selectively short-circuiting two terminals effectivelyconnected with both of the said sources.

13. In a combination radio-phonograph system, a modulated carrier wavereceiver including an electron tube having at least an anode, a cathode,and a grid electrode adapted to control the space current thereof, asource of blocking potential connected between said cathode and gridelectrode through a resistance, said source being capable of biasingsaid grid electrode beyond cut-off thereby rendering said electron tubecuits, circuit means connecting the audio input of said amplifierthrough a blocking condenser with the said source of blocking potential,audio utilization means connected to the audio output circuit of saidamplifier, power supply means common to both said amplifier and saidreceiver, a generator of audio currents, and selective switch meanshaving a single operating control member movable to a plurality ofpositions, in one of which positions said generator is operativelyconnected with the, audio input circuit of said amplifier, and inanother of which positions said source is rendered incapable ofrendering said electron tube ineffective.

14. The combination as defined in claim 13, further characterized inthat the said power supply means is provided with an impedance connectedbetween the cathode of the said electron tube-and the negative terminalof the said means, and thatv a portion at least of the potential dropacross the said impedance provides the said source of blockingpotential.

and said cathode, means for selectively receiving modulatedhigh-frequency currents, a generator of audio currents, a condenser, aninductance coupled with said last-mentioned means and effectivelyconnected between said control-grid electrode and one terminal or saidcondenser, and

selective switch means and circuits associated therewith capable ofeflectively connecting either the other terminal of said condenser withsaid cathode or said generator therebetween, whereby said condenserserves alternately as by-pass for said high-frequency currents or asaudio coupling means between said generator and said control-gridelectrode.

1'1. The combination as defined in claim 16, further characterized inthat a high-frequency output circuit is connected between the plateelectrode and cathode of the said electron tube, and an audio-frequencyoutput circuit is fixedly connected between the auxiliary electrode andcathode of the said electron tube.

18. The combination as defined in claim 16, further characterized inthat means are provided for automatically disabling the said means forselectively receiving modulated high-frequency currents when the saidselective switch means eflectively connects the said generator of audioreproduced by said loud-speaker or recorded .by said recording unit,depending upon the position of said multi-position switch.

. frequency input circuit and a generator of audio currents between thesaid other terminal of the tem, a radio receiver including an audioampli-- fier and a loud-speaker, a recording unit, a multipositionswitch and circuit means associated therewith whereby in one position ofsaid switch the said systemis conditioned to audibly repro-' ducereceived radio programs, and in another position of said switch the saidsystem is adapted to record received radio programs, a microphone, acasing carrying said microphone, a single-pole switch supported by saidcasing, a single-circuit cable connecting said microphone and saidsingle-pole switch with said radio receiver and audio amplifier, andmeans whereby operation of said single-pole switch simultaneouslydisables the said radio receiver and operativeiy connects saidmicrophone with the input circuit 0! the said audio amplifier,'therebypermitting sounds picked up by said microphone to be either audiblycurrents effectively connected in series relation betwen thecontrol-grid and cathode of said second electron tube, a volume-controlresistor having terminal connections, a tap connection intermediate itsterminals, and a variable connection movable between its terminals andpast the tap connection, a modulation detector having a carrierfrequency input circuit connected between the plate and cathode of saidsecond electron tube and an audio output circuit eflectively connectedbetween the terminal connections of said volume-control resistor, anaudio utilization circuit connected between the variable connection andone terminal connection of said volumecontrol resistor, an audiotransmission circuit including a condenser and resistor connectedbetween the auxiliary electrode of said second electron tube and the tapconnection of said volumecontrol resistor, a connection between theoathode of said second electron tube and the said one terminalconnection of said volume-control resistor, and selective switch meansand circuits associated therewith capable of simultaneously renderingineffective said source of blocking potential and said generator ofaudio currents.

JACOB H. REBOLD.

